1. Field of the Invention
The present invention relates to a method of obtaining a gas-introduced fiber-reinforced resin injection molding and to a molding obtained by the same. Particularly, the invention relates to a method of obtaining a lightweight gas-introduced fiber-reinforced resin injection molding containing glass fibers or like fibers and having excellent rigidity and strength and improved surface properties, as well as to a molding obtained by the method.
2. Description of the Related Art
Conventionally, there have been known fiber-reinforced resin moldings reinforced through the containment of fibers such as glass fibers. Because of excellent mechanical characteristics such as tensile strength, rigidity, and heat resistance, fiber-reinforced resin moldings are widely applied to automobile parts, such as inner panel, bumper absorbers, door steps, roof racks, rear quarter panels, and air-cleaner cases, as well as building and civil engineering members, such as outer wall panels, partition panels, and cable troughs.
Such fiber-reinforced resin moldings may be manufactured by the injection molding method in which a molten resin containing fibers is injected into the interior of a die. The injection molding method can manufacture a molding having a complicated shape and can advantageously mass-produce moldings having the same shape, since a predetermined molding cycle can be repeated continuously.
When a fiber-reinforced resin molding is to be manufactured by injection molding, the approach of increasing the amount of fibers in an attempt to improve the strength and rigidity of a molding tends to increase the weight of a manufactured molding. Accordingly, there is proposed the expansion injection molding method in which a foaming agent is mixedly added to a material resin in order to reduce the weight of a molding and the mixture is subsequently expanded and molded (Japanese Patent Application Laid-Open (kokal) No. 7-247679, etc.).
However, the expansion injection molding method has involved difficulty in obtaining a sufficient expansion ratio even when a foaming agent is used in a relatively large amount.
Further, even when a sufficient expansion ratio is obtained, foaming impairs the appearance of the resultant molding. Also, even though reinforcing fibers are contained, a molding sometimes fail to attain satisfactory mechanical characteristics such as strength, rigidity, and impact resistance due to a large hollow formed therein.
Thus, in order to attain a reduction in weight of a molding while maintaining required mechanical characteristics, such as strength, rigidity, and impact resistance, as well as required appearance quality, the following expansion molding methods (1) and (2) have already been proposed.
(1) An expansion molding method of obtaining a lightweight molding from fiber-containing resin pellets, which contain relatively long fibers, through the expansion of a molten resin effected by a springback phenomenon of the contained fibers.
(2) An expansion molding method of obtaining a more lightweight molding from the fiber-containing resin pellets mentioned above in (1) mixed with a foaming agent through the foaming-agent-accelerated expansion of a molten resin.
These methods can be used to manufacture a sufficiently lightweight fiber-reinforced resin molding without impairment of mechanical characteristics.
However, the molding methods mentioned above in (1) and (2) involve the following problems a) and b), respectively.
a) In order to prevent the breakage of reinforcing fibers as much as possible, an injection molding machine and a molding die must use a nozzle, sprue, runner, gate, and like elements designed specially to have a molten-resin flow passage of a diameter larger than a normally used diameter. Further, the shape of a molding is limited so as to prevent the breakage of reinforcing fibers during molding, and a die must also be designed accordingly.
Also, when an expansion ratio is increased in order to reduce the weight of a molding, pores present in a molten resin increase in size after the expansion of the molten resin, causing a reduction in thermal conductivity of the molten resin.
Accordingly, when a molded molten resin undergoes cooling, the interior of the molten resin cools considerably slower than does the surface of the molten resin. Consequently, it takes time until the entire molten resin cools. Even when the molten resin is expanded through the utilization of a springback phenomenon, the entire resin undergoes thermal shrinkage since the surface temperature of the resin drops. As a result, a sink mark is likely to be formed on the surface of a molding.
b) A gas generated from a foaming agent during molding enters between the surface of a molten resin and the molding surface of a die, often causing the formation of silver mark on the surface of a molding. Further, the gas ejected to the exterior of the die raises a problem of unfavorable odor.
Also, as a result of the residual pressure of the gas generated from a foaming agent, a molten resin is apt to ooze down from the nozzle of an injection apparatus when the nozzle is separated from the die. That is, a drawing phenomenon is likely to occur.
Further, as in the problem mentioned above in a), because of the presence of pores and bubbles within a molten resin, cooling the entire molten resin takes time. Thus, even when the molten resin is expanded through the utilization of a springback phenomenon, a sink mark is likely to be formed on the surface of a molding due to thermal shrinkage.